(1) Field of the Invention
The present invention relates to non-destructive testing and particularly to the non-invasive examination of soft tissue and body organs. More specifically, this invention is directed to medical ultrasonic equipment and particularly to pulse-echo body scanners having improved output signal resolution and/or capable of being focused at variable depths with the examining beam energy content being varied with depth of focus. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in diagnostic medicine. For a general discussion of ultrasonic diagnostic techniques and instruments, reference may be had to U.S. Pat. No. 4,137,777 which is assigned to the assignee of the present invention. The disclosure of said U.S. Pat. No. 4,137,777 is incorporated hereby by reference.
The aforementioned patent discloses an ultrasonic body scanner wherein the examining beam may be focused about a natural focal length; the natural focal length being determined by the geometry of the beam generating transducer. Thus, in accordance with the referenced patent, when an area of particular interest is noted during an examination, enhanced detail of that area may be obtained by rescanning and changing the focal length of the beam so that the examining beam will be focused at different depths within the patient's body. The variation in focus of the examining ultrasound beam is accomplished electrically and, in accordance with one embodiment, the transducer is of the single crystal coaxial array type. By way of example, the transducer may comprise a piezoelectric crystal shaped to have a natural focal length and having a first surface thereof coated with a conductive material and a second opposite surface thereof provided with a plurality of coaxial annular electrodes. The apparatus disclosed in the referenced patent employs the same crystal for beam generation and as a receiver which transduces echoes from within the test body into electrical signals for subsequent processing and display.
The apparatus and method of referenced U.S. Pat. No. 4,137,777 was a significant step forward in the art of diagnostic ultrasonics. Nevertheless, there has been a desire to provide enhanced image quality by controlling both the delivery of ultrasonic energy to a patient and by improving the processing of signals commensurate with received echoes. There has also been a desire to minimize the energy content of the pulses of ultrasonic energy delivered to a patient to a level commensurate with an acceptable echo related signal-to-noise ratio. Since signal attenuation is a function of path length, those prior art ultrasonic devices which were capable of being varied in focal length selected a beam energy content which would permit usable information to be derived from the maximum depth of focus. This, of course, resulted in excessive energy at lesser depths of focus.
In the prior art, when using either a transducer array or a single transducer with an electrode array thereon, it has been common practice to sum all of the electrical signals provided by the transducers or transducer in response to received echoes in a conventional summing circuit. That is, signals commensurate with the echoes received at the transducer or transducer array were directly coupled, via a resistive network, to the signal processing circuitry. This form of signal coupling results in comparatively high attenuation and thus prior ultrasonic diagnostic apparatus have been characterized by relatively poor signal-to-noise ratios.